In connection with vehicles, for example passenger cars, different kinds of alarm systems are used to detect burglary or attempted theft of the vehicle. If such a situation is detected, the alarm system normally emits an acoustical and/or optical alarm signal in order to attract attention so that the burglary or theft of the vehicle can be prevented. The alarm signal can also be combined with the actuation of a device which is arranged to prevent the vehicle from being started.
In alarm systems for vehicles, the use of so-called space sensors is known, i.e. sensors which detect possible movement of people or animals within a certain area. This area, in the application in question, corresponds to the passenger compartment of the vehicle. Such space or motion sensors are previously known, and can be in the shape of, for example, ultrasound sensors or microwave sensors. A space sensor which is intended for use in connection with vehicles is normally connected to a computer-based alarm control unit which is arranged to emit an alarm signal if the sensor detects movement within its area. In the event of an alarm, the alarm control unit can also be arranged to prevent the vehicle from being started, for example by deactivating the starter motor of the vehicle.
In some situations there is a need for an alarm system for vehicles which comprises a sensor and in which the alarm system is automatically activated when the driver leaves the vehicle. This function is normally required for reasons of comfort, so that the driver for example does not need to activate a switch in order to activate the alarm function. In order to achieve this automatic activation, switches in the doors of the vehicle are normally used, which switches are connected to the alarm control unit. In this manner, the alarm control unit can determine whether the doors are open or closed. The alarm control unit can detect the fact that at least one door has been opened, and subsequently that all of the doors have been closed, which is taken to mean that the driver has left the vehicle. Following this, the alarm can be activated. If motion is subsequently detected in the passenger compartment of the vehicle by means of the sensor, the alarm will be triggered.
In the above-described system, there is normally is an automatic delay in activation of the alarm, so that it is activated a certain time after the driver has left the vehicle. This means that when the last door has been closed, the alarm control unit will initiate a time delay of, for example, 30 seconds. When this period of time has passed, the alarm is activated and will be triggered as normal if the sensor detects movement in the passenger compartment of the vehicle.
In connection with the above-mentioned alarm system, there is a problem in those cases where the driver leaves the vehicle at the same time as, for example, a child or a pet is left in the vehicle. In such a case, the sensor will trigger immediately after the above-mentioned time delay has expired, which of course inconveniences those who are in the vehicle. Furthermore, the degree of attention which normally should follow a true alarm situation decreases. In addition, alarm signals, and in particular unnecessary false alarms, are a nuisance within populated areas since they disturb people in the vicinity of the vehicle.
In order to solve the above-mentioned problem, the alarm system can be equipped with some sort of control or switch which is connected to the alarm control unit, and by means of which the alarm can be deactivated when the driver leaves the vehicle, and for example a child or a pet is left in the vehicle. However, such a solution will always cause a problem since there is a risk that the driver will forget to operate this switch in order to deactivate the alarm when he intends to leave the vehicle and somebody has been left in the vehicle. On the other hand, there is a risk that the driver--if he has remembered to deactivate the alarm at a certain point in time--instead forgets to reactivate the alarm using the switch, thus causing the alarm not to be activated the next time this is necessary.